This research is designed to quantitate the permeability characteristics of dentin both in vivo and in vitro. Using dogs, teeth are prepared to expose dentin. A chamber device is cemented on the tooth and filled with 125I containing buffer. Timed blood samples are removed for counting. At the end of the experiment the tooth is removed from the dog and the experiment repeated in vitro over a fraction collector. Several experimental variables examined include the thickness of dentin, the size of the permeating molecule, the influence of filtration versus diffusion and the effects of iontophoresis on enhancing the rate at which substances diffuse through dentin. Scanning electron microscopic examination of the dentin both before and after acid etching with citric acid are correlated with the physiologic data to gain futher insight into the mechanisms of dentin permeation.